The Narcissistic Personality Disorder (NPD) is a disease. It is defined only by and in the Diagnostic and Statistical Manual (DSM). All other "definitions" and compilations of "criteria" are irrelevant and very misleading.

People go around putting together lists of traits and behaviors (usually based on their experience with one person who was never officially diagnosed as a narcissist) and deciding that these lists constitute the essence or definition of narcissism.

People are erroneously using the term "narcissist" to describe every type of abuser or obnoxious and uncouth person. That is wrong. Not all abusers are narcissists.

Only a qualified mental health diagnostician can determine whether someone suffers from Narcissistic Personality Disorder (NPD) and this, following lengthy tests and personal interviews.

It is true that narcissists can mislead even the most experienced professional (see the article above). But this does not mean that laymen possess the ability to diagnose mental health disorders. The same signs and symptoms apply to many psychological problems and differentiating between them takes years of learning and training.

(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase the print book, or 16 e-books, or 2 DVDs with 12 hours of video lectures on narcissists, psychopaths, and abuse in relationships: http://www.narcissistic-abuse.com/thebook.html)